In stationary gas turbines and gas turbines of aircraft engines, components coated with a heat insulation layer or TBC (thermal barrier coating) are employed. Components of this type are mostly cooled from inside or from one side and are heat-insulated on the hot side by a ceramic TBC layer. A low thermal conductivity is advantageous in respect of the required cooling air and thickness of the TBC layer. The ceramic TBC layer consists, for example, of zirconium dioxide stabilized with yttrium oxide (YSZ) or of other ceramic materials and stabilizers and can be applied, with or without an adhesion promoter layer, to the surface of metallic substrates by various methods, such as, for example, air plasma spray (APS), electron beam physical vapor deposition (EB-PVD), etc.
Components of gas turbines are often exposed to extreme cyclic alternating thermal stresses. During the heating and the cooling of the components, thermal stresses arise due to different coefficients of thermal expansion of the various materials and they can lead to the material structure being damaged or failing. During operation, the components age as a function of the thermal load upon the component and, after a certain operating time under temperature, are reconditioned or scrapped. In reconditioning, typically, the TBC coating and an adhesion promoter layer which is present, if appropriate, are removed. Subsequently, after further work steps, such as the pretreatment of the metal substrate and the application of a new adhesion promoter layer (bond coat), the component is coated with a new heat insulation layer again and put to use.
Reconditionability is determined by the component boundary conditions, such as, for example, the operating time under high temperature, the manufacturer of the coating and the coating quality. Since, conventionally, commercially available YSZ (Yttria-Stabilized Zirconia) is applied to the component surfaces to be coated, using standard coating methods, components with renewed heat insulation layers do not generally make it possible to draw conclusions as to the manufacturer of the coating, the relevant process parameters and the material quality. Experience shows that, even with the same initial material for the YSZ coating, considerable differences arise with regard to the useful life of TBCs. Moreover, if the boundary conditions predetermined by the coating and the operation of the component are not known, the risk of a misjudgment of the component state increases.
Dopings of YSZ coatings are known per se and are used for various purposes, such as, for example, an infiltration of Cr2O3 as corrosion protection, or dopings with Sc2O3, Yb2O3, Nd2O3, Sm2O3, Gd2O3, NiO and Cr2O3 for reducing the thermal conductivity of heat insulation layers.
A stabilized ZrO2 ceramic reinforced with hard material, having mechanical stability under hydrothermal stress, is known from DE-A-197 33 700. The ZrO2 ceramic contains, in addition to a hard material component (for example TiC), Al2O3 and, as a remainder, a stabilized ZrO2 phase with a solid solution of Y2O3 and CeO2. To produce the ZrO2 ceramic, a ZrO2 powder containing Y2O3 is provided with a Cer doping and is then mix-ground together with the hard material component and with Al2O3 in a liquid.